1. Field of the Invention
The present invention relates to an optical system and, in particular, an optical system suitable for optical apparatuses, such as silver-halide film cameras, digital still cameras, video cameras, digital video cameras, telescopes, binoculars, projectors, and copying machines.
2. Description of the Related Art
Optical systems used for optical apparatuses, such as digital cameras and video cameras, need to have a short total lens length (optical length: the length between the first lens surface on the object side and the image plane) and the short length of the optical systems. In general, as the size of an optical system decreases, the aberrations and, in particular, the axial chromatic aberration and the chromatic aberration of magnification of the optical system significantly increase. Thus, the optical performance of the optical system decreases.
In telephoto optical systems having a short total lens length, as the focal length increases, chromatic aberration increases. A telephoto optical system that corrects the chromatic aberration by using an anomalous partial dispersion material is described in, for example, U.S. Pat. Nos. 4,241,983, 4,348,084, and 6,115,188.
In addition, retrofocus optical systems are known that have a short focal length and a long back focus of the optical system. In a retrofocus optical system, a lens group having a negative refractive power is disposed in the front portion of the optical system (on the object side for photo-taking lens systems, such as cameras, and on a screen side (an enlargement side) for projection optical systems, such as projectors). In addition, a lens group having a positive refractive power is disposed in the rear portion of the optical system (on the image side for photo-taking lens systems, such as cameras, and on an original image side (a reduction side) for projection optical systems, such as projectors). Using such a structure, an optical system having a long back focus can be achieved.
However, retrofocus optical systems have an asymmetric refractive power arrangement with respect to an aperture stop. Thus, a negative distortion aberration (barrel distortion aberration) and chromatic aberration of magnification tend to occur. To correct the chromatic aberration of magnification of retrofocus optical systems, an anomalous partial dispersion material, such as fluorite, is used for a lens group on the reduction side, in which a height at which a paraxial chief ray passes through the lens surface from the light axis is relatively high (refer to, for example, Japanese Patent Laid-Open Nos. 06-082689 and 2002-287031).
In addition, the chromatic aberration of magnification of optical systems can be corrected using a liquid material having a high dispersion characteristic and an anomalous partial dispersion characteristic (refer to, for example, U.S. Pat. Nos. 4,913,535 and 5,731,907).
Furthermore, the chromatic aberration of magnification of optical systems can be corrected using a solid material composed of a mixture of a transparent material and indium tin oxide (ITO) fine particles dispersed therein (refer to, for example, U.S. Pat. Nos. 7,136,237, 7,057,831, and 7,116,497).
Still furthermore, the chromatic aberration of magnification of optical systems can be corrected using a solid material composed of a mixture of a transparent material and TiO2 fine particles dispersed therein or a solid material made of resin (refer to, for example, U.S. Pat. Nos. 7,193,789, 7,164,544, and 2007/0014025).
The optical systems using fluorite for the optical material and having a relatively large lens length can be easily corrected for chromatic aberration of magnification. However, as the lens length decreases, occurrence of chromatic aberration of magnification significantly increases. It is difficult to sufficiently correct such a chromatic aberration. This is because chromatic aberration occurring in a front lens unit of a telephoto optical system having a positive refractive power or chromatic aberration occurring in a lens unit of a retrofocus optical system disposed closer to the reduction side than the aperture stop and having a positive refractive power is simply reduced by using a low dispersion characteristic and an anomalous partial dispersion characteristic of the material of the lens, such as fluorite.
For example, for a fluorite lens using low-dispersion glass having a large Abbe number, in order to correct chromatic aberration that is increased by reducing the length of the optical system, the refractive power of the lens surface needs to be significantly changed. However, if the refractive power of the lens surface is significantly changed, a variety of aberrations, such as spherical aberration, coma aberration, and astigmatism aberration, occur. Accordingly, it is difficult to correct chromatic aberration and other aberrations at the same time. In addition, the fabrication of a glass material having an anomalous partial dispersion characteristic (such as fluorite) is significantly difficult. Furthermore, since the surface of the glass material is easily damaged, the usage of the glass material is limited for some parts of the optical system.
Since the materials described in U.S. Pat. Nos. 4,913,535 and 5,731,907 are liquid, a structure to enclose the materials is needed. Thus, when these materials are used for an optical system, the fabrication of the optical system is difficult. In addition, characteristics, such as the index of refraction and dispersion significantly vary with a change in temperature, and therefore, the resistance to the surrounding environment is not sufficient. Furthermore, since the interface with air cannot be obtained, it is difficult to sufficiently correct chromatic aberration.
The transmittance of the solid material having an anomalous partial dispersion characteristic by dispersing ITO fine particles or TiO2 fine particles in a transparent material is relatively low, compared with a widely used optical material. To prevent a decrease in transmittance of the entire optical system, it is desirable that the thickness of the solid material in the light axis direction is reduced. However, in order to sufficiently correct chromatic aberration using the solid material, a certain thickness is required. As the thickness of the solid material in the optical path increases, variation in the optical performance increases in the use environment. Thus, the resistance to the surrounding environment deteriorates. In addition, it is difficult to mold a thick solid material. Accordingly, the fabrication of an optical system is not easy. Therefore, when an optical element composed of a solid material having an anomalous partial dispersion characteristic is used for a lens or a layer having a refractive power in an optical system, it can be useful that chromatic aberration is corrected while reducing the thickness of the optical element in the light axis direction.